FIG. 7 is a view schematically showing the configuration of a laser display in the related art described in detail, for example, in Non-Patent Document 1. Light beams from laser light sources 101a, 101b and 101c for three colors, RGB, are combined by dichroic mirrors 102a and 102b and mirror 107, sent through a light collection lens 109, and scanned in the horizontal direction by a polygon scanner 104 and in the vertical direction by a galvanometer scanner 105 to be irradiated onto a screen 108. In this instance, a video is displayed on the screen by modulating intensity by light modulators 106a through 106c according to an input video signal. For example, in order to display a moving image corresponding to an NTSC video signal, about 500 scan lines in the horizontal direction are displayed for 30 frames per second, and the number of horizontal scan lines in total is 15,000 per second. This can be achieved by rotating a polygon scanner having 30 faces at 30,000 rpm. The galvanometer mirror is oscillated to reciprocate in the vertical direction 30 times per second. The resolution in the horizontal direction is determined by a modulation rate of the light modulators with respect to the scan rate. For example, in order to obtain the resolution comparable to 500 TV lines in the horizontal direction at the scan rate specified above, a bandwidth of about 10 MHz is necessary on the basis of 500×15,000=7,500,000. Such a bandwidth can be achieved with a light modulator using the acousto-optic effect or a light modulator using the electro-optic effect.
The display configured in this manner is characterized in that it can display a sharp image having high color purity by using laser light sources having adequate wavelengths because light beams from the respective light sources for RGB are monochromatic light. Sharp color display of each monochromatic light can be achieved, for example, by using a krypton ion laser having a wavelength of 647.1 nm as the red light source, a helium-cadmium laser having a wavelength of 441.6 nm as the blue light source, and a second harmonic of a neodymium-doped YAG laser having a wavelength of 532 nm as the green light source.
The display configured as shown in FIG. 7, however, has a problem of so-called speckle noises resulting from the use of a highly coherent laser light source as the light source. The speckle noises are microscopic irregular noises induced by mutual interference of scattered light from the respective portions on the screen 108 as a laser beam scatters on the screen 108. The screen 108 has a random surface shape, and a laser beam scattered on the screen causes interference due to a microscopic concavoconvex shape, and generates a microscopic bright-dark pattern depending on a viewing direction. This pattern results in the speckle noises.
In the related art, the speckle noises are removed by oscillating the screen 108. This technique uses the fact that the speckle pattern changes as the interference state on the screen varies from time to time because the position of the screen keeps changing. The speckles do not disappear in every moment; however, the pattern changes at a high speed due to oscillation, and the viewer acknowledges the resulting time-mean pattern. The viewer therefore views an image as if the speckles had disappeared. The method of oscillating the screen as described above can indeed remove the speckles effectively from a viewed image; however, there is a need to use a special screen that can be oscillated. This raises a problem that a fixed wall surface, for example, cannot be used as the screen without any restraint.
Non-Patent Document 1: Baker et al., “A large screen real-time display technique”, Proc. Society for Information Display 6th Nati'l Symp., 85-101 (1965)